1. Field of the Invention
The present invention relates to frequency division multiplex communications systems in general and to multiple subscriber communications systems in particular, in which simultaneous signalling may be present in the same frequency band as communications without interference therebetween. The present invention also relates to FDM double sideband telecommunications system having improved signalling, addressing and communications supervisory capability and more efficient frequency band utilization and increased receiver sensitivity.
2. Description of the Prior Art
A full duplex communications system of the prior art is described by U.S. Pat. No. 3,809,815 entitled "Communication System Utilizing Frequency Division Multiplexing to Link a Plurality of Stations Each Containing a Switchable Synthesizer", wherein the use of a digital phase lock loop synthesizer is described for generating a series of frequencies which may be utilized in a single sideband communications system, thereby enabling such system to be operable as a switch telephone system. In accordance with the technique described by this patent, single sideband transmission and reception and frequency division multiplexing are used to provide channelization. A bandpass filter in the transmitter provides the necessary inversion of transmitting and receiving frequencies requisite in establishing full duplex communications. Either an upper or lower sideband generated as the output of a balanced modulator is selected automatically.
U.S. Pat. No. 3,809,816 entitled "Communication System Utilizing Frequency Division Multiplexing and a Frequency Plan Therefor" describes the generation of a frequency pattern for providing the requisite transmitting and receiving frequency inversion described with reference to U.S. Pat. No. 3,809,815. U.S. Pat. No. 3,809,816 further describes the relationship between the transmitter intermediate frequency and the frequency transmission band required for system operation.
While neither of the above patents describe apparatus or method for transmission of signalling data required for establishing communications, such signalling data may be transmitted therein either by an in-band signalling system, wherein signalling is performed before and after communications are established, or by an out-of-band system. In such an out-of-band system, a guard band between channels wherein a limited out-of-band signalling filter assembly could be utilized with some limited signalling being achieved. The above-described prior art patents due to their lack of convenient channels for signalling and supervision, do not readily permit system users to perform impromptu conferencing. The described patents use direct synchronous detection for reception which requires that all amplification and filtering occur at the audio or baseband frequencies. It is thus apparent that a folding of frequencies about the zero frequency point occurs requiring that twice the needed channel frequency space be allocated for any communications, since the upper and lower sidebands around the virtual carrier are folded one upon the other. The described prior art patents avoid this by transmitting only a single sideband, either upper or lower, with the opposite sideband not being transmitted. This technique results in a doubling of the noise power and in a reduction of the signal-to-noise ratio of the transmitted signal. In addition, the amplification of the received signal at baseband results in amplifier generated noise. These deficiencies result in a lower limit for the amplitude of the received signal, the sensitivity of the receiver and the maximum amplification available, and also require that filters be designed for operation at audio frequency with the consequences that such filters are substantially larger than similar filters designed for operation at other frequencies.
The present invention and the incorporation thereof in a telecommunications system eliminates the prior art requirements for high gain audio amplification, eliminates the amplifier noise (l/f noise) previously inherent in the prior art baseband frequency amplifiers, eliminates the doubling of noise power due to the frequency folding around the zero frequency and allows the use of smaller and less expensive filters. Thus, both receiver sensitivity and transmitter efficiency are substantially improved with a consequent increase in the signal-to-noise ratio which enables a lengthening of the transmission path over which signals may be transmitted and/or a decrease in the requisite signal power.
A further advantage of the present invention is the elimination of crossover intermodulation products of low order due to the greater flexibility in selection of reference frequencies from the communications band and the consequent elimination of the extremely well-balanced mixers of the prior art. The present invention more effectively utilizes the frequency spectrum of the communications channel than do systems of the prior art in that both upper and lower sidebands of the communications frequency are used simultaneously.
In accordance with a further aspect of the present invention, the generated sidebands are utilized such that communication occurs at one of the two available sidebands, while the other sideband is available for signalling, monitoring, break-in and other functions, such as deriving a busy signal and handshaking in telecommunication systems.
A further advantage of the present communications technique over the prior art is the utilization of one output/input mixer and the duplexing of a single bandpass filter to both reduce the electrical components previously required in each subscriber set as well as the number of taps previously required to be made to gain access to the transmission line in a multiple subscriber telecommunications network which results, due to the difference frequency between transmitter and receiver intermediate frequencies, being substantial, i.e. greater than 50% apart and at radio frequencies as contrasted to the present in systems of the prior art. By the implementation of the present invention and its resultant utilization of less critical balanced modulator and amplifier requirements, greater use of integrated circuit technology may be achieved by integrating substantial system portions in single chip form, thereby further reducing subscriber costs when such systems are produced in large quantity.